The present invention relates to a web which is suitable for use as a fluid transport mechanism. In particular, the present invention relates to fast blooming surfactants that co-operate with other components of the web to enable a surface energy gradient which facilitates fluid transport in a preferential direction from one surface toward another surface and resist fluid transport in the opposite direction.
It has long been known in the field of disposable absorbent articles that it is extremely desirable to construct absorptive devices, such as disposable diapers, sanitary napkins, incontinent briefs, bandages, wound dressings, and the like, presenting a dry surface feel to the user to improve wearing comfort and to minimize the potential for development of undesirable skin conditions due to the prolonged exposure to moisture absorbed within the article. Accordingly, it is generally desirable to promote rapid fluid transfer in a direction away from the wearer and into a retentive structure, while resisting fluid transfer in the reverse direction.
One viable prior art solution to the aforementioned problem has been to utilize a covering or topsheet on the exposed, wearer-contacting surface which comprises a web of formed, apertured thermoplastic film. Commonly assigned U.S. Pat. No. 4,342,314, issued to Radel et al. on Aug. 3, 1982, the disclosure of which is hereby incorporated herein by reference, discloses a representative formed film of this variety. Such webs utilize capillary fluid transport to conduct fluid away from one surface (wearer-contacting) into and through the web via three-dimensional capillaries formed into the material, and then into the underlying absorbent structure. In order to address consumer concerns with regard to plastic-like appearance and feel, webs of this variety have been developed which include an interconnected structure of fiber-like appearance in the interest of generating a more clothlike, aesthetically-pleasing appearance. In addition, apertured, formed thermoplastic film webs have been developed which further include microscopic surface texturing (microtexture) and/or microscopic apertures (microapertures) to further enhance the visual and tactile impression of such webs. Representative film webs of this variety are discloses in commonly assigned U.S. Pat. No. 4,463,045, issued to Ahr et al. on Jul. 31, 1984, and U.S. Pat. No. 4,629,643, issued Dec. 16, 1986 to Curro et al., the disclosures of which are hereby incorporated herein by reference.
Another viable prior art solution has been to utilize a fibrous material as a covering or topsheet on such articles, alone or as an overlay or laminate over other materials. A representative topsheet structure of this variety is disclosed in Statutory Invention Registration H1670 published in the name of Aziz et al. on Jul. 1, 1997, the disclosure of which is hereby incorporated herein by reference. Such fibrous materials may take the form of a woven or nonwoven web of a suitable fiber variety, and may or may not include discretely formed apertures in addition to the inherent porosity of the web itself Webs of this variety also exhibit capillary fluid transport characteristics via the three-dimensional capillaries formed by inter-fiber spaces, likewise conducting fluid away from the wearer-contacting surface and into the underlying absorbent structure. Such webs exhibit an aesthetically-pleasing, cloth-like surface appearance and tactile impression due to the fibrous nature of the surface.
While capillary webs of the foregoing varieties are effective in transporting fluid, their effectiveness is limited in that such capillary structures can only move fluid once it reaches the capillary interior. Fluid which wets and remains on wearer contacting surfaces contributes to a xe2x80x9cwetxe2x80x9d tactile feeling or impression, and to the extent that such fluid may be colored or opaque also contributes to a xe2x80x9cstainedxe2x80x9d visual impression. Surface textures naturally occurring in the material of the web or imparted thereto in formation further increase the likelihood that residual fluid will be trapped or retained on the wearer-contacting surface rather than entering capillary structures for transport away from the surface. Thus, surface topographies which contribute to desirable visual and tactile impressions when dry can also tend to retain residual fluid on the exposed surface and thus reduced desirability under in-use conditions.
Such wetting is further exacerbated by the use of surfactant materials to encourage flow of bodily fluids on the surface of such webs. The use of surfactant materials is described, for example, in U.S. Pat. 4,535,020, issued in the name of Thomas, et al. on Aug. 13, 1985 which describe incorporating a surfactant material into the polymer blend used to produce a perforated film material. The perforated films described therein are said to have much higher liquid transmission rates than similarly made non surfactant treated perforated films. Similarly, U.S. Pat. No. 5,520,875, issued in the name of Wnuk, et al on May 28, 1996, describes incorporating a surfactant into a coextruded multilayer polymeric film that is formed into an apertured web.
The art has also discussed various surfactants as being useful for incorporation into polymeric matrices. For example U.S. Pat. No. 4,070,218, issued to Weber on Jan. 24, 1978, discloses incorporating nonionic surfactant materials into the polymer that is extruded to form a nonwoven web. The materials are said to be useful as lubricating agents for softening the web. The reference further teaches heating the web to cause the material to migrate to the surface. While such materials may be suitable for softening the web, webs using such materials would have a hydrophilic surface and would still suffer the same deficiencies with fluid retention that are discussed above. Further, the reference teaches the necessity of an added heating process step to cause the materials to come to the surface.
U.S. Pat. No. 5,439,734, issued to Everhart and Meirowitz on Aug. 8 1995, also teaches the necessity of post extrusion heat activation. Further, the surfactant material that is described most completely has an unsaturated hydrophobe. As is known, such unsaturated materials may be oxidatively unstable, producing degradation products having unpleasant odors.
U.S. Pat. No. 4,578,414, issued in the names of Sawyer and Knight on Mar. 25, 1986, teaches surfactants having a high degree of ethoxylation. As is known, a high degree of ethoxylation will require a high molecular weight hydrophobe in order to maintain a mid-range HLB (desirable for wetting performance). Such molecular weight increases can lead to undesirable slow blooming.
U.S. Pat. No. 4,923,914, issued to Nohr and McDonald on May 8, 1990 and European Patent Application EP 683,260 A2, published in the names of Yahiaoul, Perkins, and Jascomb on Nov. 22, 1995 both teach the desirability of using surfactant materials that are very efficient wetting agents. In particular, the references teach the desirability of siloxane-based and/or fluorocarbon-based hydrophobes. While such surfactant materials may be very efficient in providing a wettable surface, if the surfactant is too efficient, it will eliminate the aforementioned surface energy gradient that has been found to be desirable for fluid transport purposes.
The art has addressed the problem of transporting fluids away from a wearer contacting surface while retaining desirable visual and tactile properties by providing pervious webs (e.g. via fluid passageways provided by apertures or interfiber capillaries) treated so as to have microscopic, discontinuous, spaced apart depositions of a low surface energy material on at least the wearer contacting surface. The low surface energy material provides a surface energy gradient between the material and the remainder of the body surface that exerts a force on any fluid contacting the body surface to direct such fluid toward and into the fluid passageways for transportation away from the body surface and into an absorbent article when such webs are used as a topsheet in an absorbent article. Webs treated so as to have such a surface energy gradient are described in U.S. Pat. No. 6,025,049 issued in the name of Ouellette, et al. on Feb. 15, 2000. While webs having a surface energy gradient have desirable fluid transport properties, surfactants known to the prior art as being preferable for increasing liquid transmission rates through untreated webs, have undesirably low permeabilities through such treated webs. In particular, it has been found that, those components of such prior art surfactants that permeate through such treated webs rapidly have insufficient surfactant activity to provide a desirable balance of acquisition and rewet reduction. Conversely, those components having sufficient surfactant activity so as to be effective permeate much more slowly. Such low permeability of preferable surfactant components results in an undesirably long delay before webs using prior art surfactant materials and low surface energy depositions have the desirable balance of fluid acquisition and rewet reduction.
Accordingly, it would be desirable to provide a web having an effective balance of enhanced effectiveness in transporting fluid away from the surface which is initially contacted by a fluid and prevention of rewetting of that surface by absorbed fluid, wherein the web is available for conversion into a finished product without a long induction period before the web has suitable fluid handling properties. More particularly, it would be desirable to provide the underlying polymeric structure that forms the base of such webs with surfactant materials with surfactant materials incorporated therein having a high permeation rate through webs treated with low surface energy materials which results in fast blooming of active surfactant components to the surface of the web so such webs are quickly available for conversion into finished absorbent articles. It would be also desirable if such surfactant materials have satisfactorily high thermal stability and/or low volatility so the surfactant material does not undesirably build up on equipment components when such surfactant materials as are incorporated into the polymer composition used to extrude the underlying polymeric structure that forms the base for webs having the above-mentioned desirable balance of acquisition and rewet reduction. Finally, it is desirable that such surfactant materials, when incorporated into a web that will ultimately be converted as a component of an absorbent article, retain the fluid handling properties after long storage periods.
The present invention comprises fluid permeable webs having a fast blooming surfactants incorporated therein and treated with a low surface energy material to create a surface energy gradient between the underlying polymeric structure of the web and spaced apart microscopic depositions of the low surface energy material. The surfactants have a permeation rate of less than 240 hours and a surface tension in an aqueous solution at a concentration that is greater than the critical micelle concentration that lies between the critical surface tension of the underlying polymeric structure of the web material and the critical surface tension of the low surface energy depositions. Preferred surfactants also have a weight loss on heating to 250xc2x0 C. that is less than about 20%. More preferred surfactants have a hydrophobic chain that is substantially saturated.